Welded joints with polymer sealant

ABSTRACT

An assembly, such as an aircraft component, is formed with a weld joint by, for example, using friction stir welding, FSW, to form a lap joint between a stiffer and the interior surface of the skin of the component. A sealant layer, such as a monomer sealant/adhesive is applied to the surfaces to be joined, and is cured in place by the elevated temperatures of the welding process to form an elastomeric fay surface sealing, such as a fluoroelastomeric coating, to protect the component from corrosion at the welds. Additional heat for curing may be applied by laser or induction heating or the like.

RELATED APPLICATIONS

[0001] This application claims the priority of provisional application60/402,505 filed August 7, 2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to surface sealants for welded joints ingeneral, and in particular, relates to polymer fay surface sealants forfriction stir welding (FSW) joints used in aircraft construction.

[0004] 2. Background of the Invention

[0005] Welded joints, such as lap joints, are used to join metal parts.Lap joints are used to join overlapping surfaces using one or more weldsat the overlap of the materials. The surfaces of each piece of joinedmetal adjacent the fasteners or welds that are mated by the lap joint,called “fay” surfaces, are often not fully bonded by the weld or otherfasteners and are often protected from corrosion by conventional faysurface sealants, such a polysulfide or polythioether, applied to themating surfaces prior to joining. Such fay surface sealants may also beused to reduce mechanical and fatigue problems resulting from rubbingbetween the fay surfaces, vibration and the like.

[0006] Fay surface sealants are used in welding to prevent or reducecorrosion in large part by reducing moisture which may be trappedbetween the fay surfaces and/or brought in by capillary action.Conventional fay surface sealants may degrade the properties of the weldand are often degraded by the heat and mechanical activities involved inthe welding and working of the materials as well as by vibration duringuse. Corrosion of fay surfaces within welds is often very difficult todetect by inspection.

[0007] Welding techniques typically use elevated temperatures to bondmetals and metal alloys. Friction stir welding (FSW) is a weldingtechnique in which the shoulder of a rotating tool is applied to thematerials to be joined to heat and soften the materials by friction. Thetool includes a rotating pin which penetrates the joint and stirs thematerials together. Solid state joints are produced thereby without theaddition of filler or the use of shielding gases. Friction stir weldingis advantageously used in aircraft construction, for example, to weld astringer or other support to the surface of the aircraft skin.Conventional FSW welding processes apply a zone of corrosion resistantmaterial to cover the welded joint during and after welding as a faysurface sealant. An example of this technique is disclosed in U.S. Pat.No. 6,045,028, Martin et al. Corrosion of fay surfaces within a FSW weldin an aircraft is a very dangerous condition because of the reduction instrength of the aircraft structure, and it is very difficult to detectby inspection.

[0008] What is needed is a new fay surface sealant technique forproducing FSW welded components with improved corrosion resistance foruse, for example, in the production of airplanes and parts of airplanes.

SUMMARY OF THE INVENTION

[0009] In a first aspect, a method of welding includes positioningsealant between surfaces to be welded together and welding at leastportions of the surfaces together to cure the sealant there between.

[0010] In another aspect, a welded structure includes a first member, asecond member welded to the first member, and a fay surface sealantbetween the first and second member cured by the welding.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a cross sectional side view of a portion of an aircraftor similar structure including a lap joint between metal surfaces,showing typical locations of FSW welding joint lines, with a polymericfay surface sealant for improved corrosion resistance.

[0012]FIG. 2 is a cross sectional side view of the structure of FIG. 1showing an alternate stiffener welded to a surface and protected with apolymeric fay surface sealant.

[0013]FIG. 3 is a cross sectional view of an aircraft wing and arepresentative stiffener with a polymeric fay surface sealant.

[0014]FIG. 4 is a cross sectional view of generic aircraft structure anda representative stiffener with a polymeric fay surface sealant.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT(S)

[0015] Referring now to FIG. 1, a cross sectional side view of a portionof an aircraft or similar structure is shown including lap joint 10between an aluminum, steel, titanium or alloy surface 12, such as anaircraft wing or body skin, with a stringer, such as an “L” shapedaluminum, steel, titanium or alloy support member 14. The lap joint isformed by using FSW or other known welding techniques at one or more ofthe welding locations generally indicated as FSW welding joint lines 16,18 or 20. These joint lines are perpendicular to the plane of thedrawing, and are formed by rotational of FSW welding tool 22 againstsupport member 14 and/or surface 12. The rotation of tool 22 causesfriction to heat and soften the materials to be joined, and mechanicalmixing of the plasticised materials to form the joint. The elevatedtemperatures from this process may polymerize a sealant and/or adhesivemonomer layer which, when cured, forms a corrosion protection layer, faysurface sealant layer 24.

[0016] The monomer layer preferably has characteristics suitable for theFSW welding process as well as suitable for the intended use of thejointed assembly. For example, the monomeric layer should be easy toapply to the critical area, and cured but not substantially damaged bythe elevated temperatures produced by the welding. When cured, sealantlayer 24 should provide corrosion resistance for the fay surfaces of thejoint by, for example, resisting the intrusion of moisture by capillaryaction. A particularly useful fay surface sealant material for sealantlayer 24, is formed by the application of a layer which when cured formsa fluoroelastomeric polymer to create a protective corrosion shieldbetween the adjacent fay surfaces 26 and 28 of support member 14 andsurface 12, respectively. It may be desirable to extend the coverage ofsealant layer 24 beyond fay surfaces 26 and 28 in many applications.

[0017] Lap joint 10 is formed by the following process. The surfaces of12 and 14 are prepared normally for FSW welding, thereafter, sealantlayer 24 is prepared for application to the surfaces to be welded. Oneparticularly useful polymeric fay surface sealant may be formed by useof a fluoroelastomer adhesive PLV 2100 available from PelsealTechnologies, LLC of Newtown, Pa. PLV 2100 is a 2 part adhesive whichmay be mixed in the ratio of 25-2₇ parts by weight of PLV 2100 basematerial to one part by weight of Accelerator #4, which is added to thebased material and mixed thoroughly for about 5 minutes. For sprayapplication, the mixed coating may then be thinned by addingmethylisobutylketone (MIBK) or methylethylketone (MEK) in a 1:1 ratio byweight. It is desirable to avoid introducing substantial amounts of airinto the mixture and it may be desirable to off-gas the mixture for 10minutes prior to the application. Other materials shown to be suitableinclude Pelseal PLV 6032 and Thermodyne THP-2000.

[0018] Additional surface treatments may also be used with the processof the present disclosure. If device components are aluminum or aluminumalloy and increased joint corrosion resistance is desired, aconventional chemical conversion coating may be applied on at least thesurfaces to be welded before application of the surface sealant.

[0019] The mixture may be applied to surface 26 of stiffener 14, and/orto surface 28 of skin 12, by using a hard rubber roller, brush, orpreferably an HPLV sprayer as an applicator 29. An appropriate adhesivewet film thickness for layer 24 may be in the order of up to about0.0005 to 0.0100 inches, preferably from 0.001 to 0.005 inches, to avoidproblems such as causing FSW tool 22 to dive during the welding process,potentially creating an unacceptable weld. After the mixture is applied,surface 26 of support member 14 may then be clamped, preferably withinabout 20 minutes, to surface 28 of skin surface 12. If the clamping isnot-accomplished-within minutes, it may be desirable to rewet thesurface by spraying with a fine mist of MIBK or MEK for up to about 1hour after coating. Thereafter, the application of a new coating may bedesirable if clamping has not been accomplished.

[0020] The PLV 2100 fluoroelastomer adhesive will cure at a temperatureof about 75° F.±10° F. in approximately 24 hours. The application ofrotating FSW tool 22 to form FSW welding joint lines 16, 18 and/or 24serves to create the elevated temperatures necessary to cause thepolymeric sealant adhesive to cure more quickly. In addition to the heatgenerated by the friction of the rotating contact between FSW tool 22and weld points line 16, 18 or 20, additional heat may be provided toreduce cure time prior to or as part of the FSW welding process by theuse of laser 30, in a process known as laser-assisted FSW (LAFSW) or byuse of a heater, such as induction heater 32. After FSW welding to formjoint 10 with polymeric fay surface sealant layer 24, the sealant may besufficiently cured to permit other work, such as priming, on adjacentnon-welded surfaces within the order of about 4 hours.

[0021] Referring now to FIGS. 2, 3 and 4, there are many different typesof stiffeners and surfaces to be joined that may be improved by the useof a sealant cured by the welding process. In FIG. 2, “Z” shapedstiffener 15 may be welded, by FSW or similar processes by lap joint orother known welding technique, to surface 12 at joints 16, 18 or 20 bythe application of a suitable adhesive or sealant which is cured to formfay surface sealant 24.

[0022] Referring now to FIG. 3, a cross section of aircraft wingassembly 34 is shown in which stiffener 36 is joined to an appropriatesurface by a welding process which cures a sealant layer to form anappropriate polymerized fay surface sealant 24.

[0023] Referring now to FIG. 4, a cross section of generic aircraftstructure 38 is shown in which Z shaped stiffener 37 is joined to anappropriate surface by a welding process which cures a sealant layer toform an appropriate polymerized fay surface sealant 24.

[0024] Although PLV 2100 fluoroelastomer adhesive was used as an exampleof a suitable material to be applied to form fay surface sealant layer24, other polymers cured but not damaged by the heat of the weldingprocess to form a layer having appropriate corrosion resistantproperties may be used. Fluoroelastomers are particularly useful forthis purposes and include Viton branded materials from Dupont/Dow.

[0025] Although FSW welding techniques are particularly useful forproducing weld joints which can be protected by fay sealants cured bythe welding process, other welding techniques may be used. Similarly, inaddition to lap joints, fillets and any other joint which may produce afay or unbonded surface area, may be protected by fay sealants cured bythe welding process. The use of fay surface sealants cured at least inpart by the welding process is particularly useful in the constructionof aircraft and aircraft sub-assemblies, many other assemblies maybenefit from the use of heat cured fay surface sealants.

What is claimed is:
 1. A method of welding comprising: positioningsealant between surfaces to be welded together; and welding at leastportions of the surfaces together to cure the sealant there between. 2.The invention of claim 1 wherein the welding further comprises: frictionstir welding.
 3. The invention of claims 1 or 2 wherein positioning thesealant further comprises: positioning a monomer between the first andsecond surfaces to be welded.
 4. The invention of claim 3 whereinpositioning the monomer further comprises: partially curing the sealantbefore welding the first and second surfaces together.
 5. The inventionof claim 4 wherein welding the surfaces further comprises: completingthe curing of the sealant.
 6. The invention of claim 3 whereinpositioning the monomer further comprises: applying an adhesive monomerto the surfaces to be welded.
 7. The invention of claim 3 whereinpositioning the monomer further comprises: partially curing the monomerbefore welding the surfaces together.
 8. The invention of claim 3wherein positioning the monomer further comprises: applying an adhesivemonomer to the surfaces to be welded, and partially curing the sealantbefore welding the surfaces together.
 9. The invention of claim 3wherein welding the surfaces together further comprises: forming asealant layer between at least portions of surfaces being welded bypolymerizing the monomer.
 10. The invention of claim 9 wherein forming asealant layer further comprises: polymerizing the monomer by welding thesurfaces together.
 11. The invention of claim 3 further comprising:applying heat to the first and second joint elements to cure thesealant.
 12. The invention of claim 11 wherein applying heat furthercomprises: applying laser energy to the surfaces to be welded to curethe sealant.
 13. The invention of claim 3 where the welding produces: alap joint or filleting.
 14. The invention of claim 3 wherein positioningsealant further comprises: applying an elastomeric sealant.
 15. Theinvention of claim 14 wherein positioning sealant further comprises:applying a fluoroelastomeric sealant.
 16. A welded structure comprising:a first member; a second member welded to the first member; and a faysurface sealant between the first and second member cured by thewelding.
 17. The invention of claim 16 wherein the second member furthercomprises: a second member welded to the first member by friction stirwelding.
 18. The invention of claims 16 or 17 wherein the fay surfacesealant comprises: a monomer layer applied to the fay surfaces of thefirst and second members before welding.
 19. The invention of claim 18wherein the fay surface sealant comprises: an adhesive monomer layerapplied to the fay surfaces of the first and second members beforewelding.
 20. The invention of claim 18 wherein the fay surface sealantcomprises: an adhesive monomer layer applied to the fay surfaces of thefirst and second members and partially cured before welding.
 21. Theinvention of claim 20 wherein the fay surface sealant comprises: anadhesive monomer layer applied to the fay surfaces of the first andsecond members before welding and completely cured by welding.
 22. Theinvention of claim 18 wherein the fay surface sealant comprises: anadhesive monomer layer between the fay surfaces of the first and secondmembers and at least partially cured by the welding.
 23. The inventionof claim 22 wherein the fay surface sealant comprises: an adhesivemonomer layer between the fay surfaces of the first and second membersand at least partially cured by heat applied thereto in addition to heatapplied thereto by the welding.
 24. The invention of claim 22 the faysurface sealant comprises: an adhesive monomer layer between the faysurfaces of the first and second members and at least partially cured bylaser energy.
 25. The invention of claim 18 comprising: a lap joint orfilleting associated with the fay surface sealant.
 26. The invention ofclaim 18 wherein the sealant further comprises: an elastomeric sealant.27. The invention of claim 14 wherein positioning sealant furthercomprises: applying a fluoroelastomeric sealant.